Power converters are widely used to provide required voltages and load currents especially in environments that have higher concentrations of telecommunications or computer equipment. These power converters are often required to provide higher levels of output power for a small physical volume, since space is often at a premium. This requirement dictates that the power converters be high power density devices. These higher power density concentrations also dictate that the power converter operate with as high an efficiency as possible to minimize the converter's heat generation and therefore its operating temperature rise to the degree possible.
Through the use of switching regulator techniques, power converter efficiencies may be achieved that are typically higher than those of linear regulation techniques. These switching regulator techniques, however, give rise to other efficiency-decreasing problems that are not typically encountered in linear regulators. Because of higher switching frequencies (50 to 100 kilohertz), parasitic circuit inductance and capacitance elements in the transformers and switching devices cause "ringing" due to circuit resonances. This ringing typically presents both device and efficiency problems that must be addressed to operate the converters at high power densities.
The use of power converters that have multiple outputs allows several output voltage and current requirements to be accommodated by one converter. This is often accomplished through the use of multiple secondary transformer windings. Typically, the switching regulator is constrained to provide active regulation on just one of the outputs. Then, regulation of other outputs depends on a voltage reflection across the transformer of the main output regulation and is usually impacted by design parameters affecting the major components, such as a transformer. For example, if a main output regulation is about five percent, a secondary output regulation may only be on the order of ten percent.
The flyback converter is a very popular power supply topology for use in low-power, multiple output applications. When the main output voltage regulation requirement is moderate, these converters may be operated with the main output being regulated by the switching regulator and the other output (s) being "cross-regulated" as discussed above. The flyback converter is also attractive since its mode of operation allows the output circuits to be simplified in that an output inductor is not required. The output filter capacitor charges to the voltage delivered by the secondary output on each switching cycle. This arrangement normally works well for higher output load currents. However, for conditions of light output load currents, ringing associated with the switching regulator becomes significant and causes peak charging of the lightly loaded outputs. This ringing causes the regulation of the secondary output to deteriorate substantially, even to the point of being unacceptable.
Accordingly, what is needed in the art is a way to provide normal regulation for secondary outputs under light load conditions.